Since its introduction for use by the general public in the early 1980's, mobile communications utilizing cellular telephone technology has experienced phenomenal growth. When initially introduced, infrastructure costs associated with establishing mobile switching offices (MSO) as well as the various base transceiver stations (BTS) providing signal coverage defining communication cells, caused cellular telephony to be expensive to the user. Likewise, these infrastructure costs demanded that cellular service providers (CSP) initially provide cellular service only in metropolitan areas where the number of potential subscribers would be maximized.
As cellular service areas have matured, and the number of subscribers have grown, CSPs have continuously expanded their service areas to include less populated areas, and even rural service areas. Today it is not uncommon, and even expected, that continuous cellular coverage will be available to subscribers on extended cross country journeys.
This proliferation of cellular service has not only caused the service to be available to ever increasing areas of the country, but has also driven down the cost of the service such that it is affordable to a large number of the population. It is common today not only for business persons to remain in touch with the office through the use of cellular telephony but, due to their affordable nature, for family members to utilize the technology to remain in touch during leisure times as well.
This wide spread use of cellular communications has been a boon to society in that instant communication is very often available during emergencies. It is not uncommon for passersby to possess a cellular telephone and notify authorities, such as through dialing a local 9-1-1 system, immediately upon the occurrence of a vehicular accident, even when such an accident occurs in a remote area. Likewise, there have been numerous occasions where a citizen has witnessed undesired behavior, such as operation of a vehicle under the influence of drugs or alcohol, and been able to notify authorities through the use of cellular telephony before disastrous consequences have transpired.
However, as most enhanced 9-1-1 (E-9-1-1) systems are unable to determine the position of a mobile phone, these reports to authorities are limited in their usefulness as they often are dependent on the caller being able to accurately describe his/her position. Often times, a caller may not know his/her position with a great deal of certainty, or in the excitement of the moment may misstate the location. Furthermore, where the caller is the victim of the tragedy, the caller may be unable to clearly speak with the authorities so contacted, or may have only time enough to dial the 911 code.
Similarly, information regarding the position of a mobile unit may be useful in the operation of the cellular system. For example, where known gaps in cellular communication coverage exist in the system, such position information may be utilized by the system to adjust resources in order to avoid interruptions in the communication or to broadcast an announcement that the service will continue when the gap is cleared. Utilization of such information to improve cellular communication is disclosed in the above-referenced co-pending commonly assigned U.S. patent application entitled "Cellular System Signal Conditioner." However, as the positioning information is utilized for system control operations, as above, it is necessary to automatically determine the mobile unit's position.
Therefore, it is advantageous for a cellular system to use a method to automatically determine a mobile unit's position. Further fueling the implementation of such capabilities are various government entities directing cellular operators to have the ability to determine the location of an active mobile communication unit to within a predetermined levels of accuracy. Such demands dictate that an accurate and reliable system be developed and deployed rapidly. However, the above described proliferation of cellular service necessitates that a great many BTS sites be adapted for such location determinations.
Current technology has utilized signal triangulation well known in the art to determine a mobile unit's location. However, such systems are by design limited to determining the position of origination of a signal in communication with three spatially diverse BTSes.
In rural areas, where demand for cellular service is not great, BTSes are typically spaced a maximum distance apart in order to provide cellular coverage for as large an area as possible with as little infrastructure cost as is possible. These widely spaced BTSes provide a minimum communication overlap, and thus reduce the possibility of a mobile system being in communication with two such BTSes, much less the three BTSes necessary for position triangulation. This problem is further exasperated by the fact that a great number of mobile units in use today are of the hand held variety which utilize lower power than vehicle mounted units. These hand held units often experience marginal signal strength even with the rural BTS nearest their position. Therefore, prior art systems for determining mobile unit position are very limited in their usefulness in such areas.
In urban areas, where numerous BTSes are distributed in relatively close proximity in order to provide increased capacity, it is often possible to detect a mobile unit's signal on the three BTSes necessary for triangulation. However, in urban environments, typically there exists large structures causing reflected signals and, thus, erroneous measurements of distance as necessary for triangulation.
A need therefore exists in the art for a system and method enabling the automated determination of position of a mobile communication unit operating within the coverage area of a single radio transceiver station.
A further need exists in the art for a system and method providing for the accurate determination of the position of a mobile communication unit, although operating within communication range of three radio transceivers, who's signal is subject to indirect communication caused by ground clutter.
A yet further need exists in the art for a cost effective, yet accurate and reliable, system which may be implemented with a minimal amount of infrastructure cost associated with its deployment.